M2M Communication
Wireless Machine-to-Machine (M2M) communication is important in a variety of applications, such as factory automation, wireless telemetry and advertising. M2M applications typically communicate very short blocks of data, with a very low probability of error and low latency.
However, the transmitted signal is subject to Rayleigh fading as the signal passes through the channel. While Multiple-Input Multiple-Output (MIMO) systems, which have space-diversity, perform well over Rayleigh fading channels, large MIMO systems are complex and difficult to implement, particularly on resource constrained devices that are typically used in M2M networks.
Thus, systems that are of interest use Single-Input Single Output (SISO) transceivers, or virtual MIMO systems with a small number of transmitters and/or receivers, which makes it difficult to take advantage of signal gains in MIMO systems.
To improve the performance for SISO and small MIMO systems, Annavajjala et al., in “Achieving near-exponential diversity on uncoded low-dimensional MIMO, multi-user and multi-carrier systems without transmitter CSI,” IEEE Information Theory and Applications Workshop (ITA), 2011, describe that blocks of symbols can be “precoded” with a near-unitary matrix with elements selected at random in a Pseudo-Random Phase Precoding (PRPP) procedure. The symbols can be decoded using Maximum Likelihood (ML) decoding. That procedure achieves exponential diversity, which means that errors are reduced exponentially with the Signal-to-Noise Ratio (SNR), similar to the decoding symbols received via an Additive White Gaussian Noise (AWGN) channel.
The computational cost involved in ML decoding is very large, and thus, a sub-optimal Iterative Likelihood Search (ILS) procedure can be used. That procedure also converges to the ML solution for infinite block sizes, and thus achieves near-exponential diversity for large blocks. In practice, that procedure works for block sizes of about 400 symbols. However, it is often necessary, particularly in M2M applications, to transmit much smaller blocks.